The goal of these studies is to determine how Brain derived neurotrophic factor (BDNF) and its receptor TrkB influence vasopressin release to a progressive physiological challenge, water deprivation. Our data suggest that BDNF may facilitate both glutamate and GABA signaling during water deprivation by causing phosphorylation NR2B and increasing the expression of the chloride transporter KCC2. Increased NR2B phosphorylation would increase NMDA receptor activation enhancing vasopressin release while increased KCC2 activity could increase the inhibitory effects of GABA representing a homeostatic synaptic compensation to increased excitation during water deprivation. Specific Aim 1: to determine the role of neurohypophysial BDNF-TrkB signaling and enhanced glutamate action through NR2B phosphorylation during sustained vasopressin release induced by water deprivation. Hypothesis: Phosphorylation of TrkB associated with water deprivation leads to enhanced glutamate activity due to phosphorylation of NR2B NMDA receptor subunits through Fyn kinase, a member of the Src kinase family contributing to NMDA mediated plasticity in MNCs Specific Aim 2: to test the hypothesis that neurohypophysial BDNF-TrkB signaling enhances the inhibitory effects of GABA during water deprivation. Hypothesis: BDNF-TrkB signaling during water deprivation increases KCC2 expression enhancing the inhibitory effects of GABA in MNCs via Src kinase and without the activation of Phospholipase C gamma. Methods: These experiments will for the first time define the roles of BDNF-TrkB signaling in the homeostatic regulation of sustained AVP release. An integrative approach will be employed that includes in vitro and in vivo electrophysiological experiments, whole animal experiments in which TrkB and Src kinase antagonists will be locally applied to the SON, and functional studies of water balance and AVP release in the rat. Benefit: These experiments will address an existing gap in our understanding of the physiological regulation of neurohypophyseal function. The findings of these experiments could potentially alter the way that inappropriate vasopressin release is studied and conceptualized clinically.